1. Field of the Invention
The invention relates to a hybrid drive apparatus for a vehicle incorporating an engine and a motor generator as power sources thereof, and more particularly, to an art for restarting an engine that has been stopped in a state where the vehicle is driven by a motor generator for reducing fuel consumption.
2. Description of the Related Art
A hybrid drive apparatus is known as a drive apparatus for a vehicle which incorporates a combustion engine (hereinafter called an "engine") and an electric motor generator (hereinafter called a "motor generator") each serving as a power source. The engine, one of the power sources, is characterized in that the decrease rate of efficiency is rapidly raised to match the decrease rate in the load in a lighter load region.
In order to reduce fuel consumption for energy saving, a drive apparatus has been disclosed which is adapted to a method of automatically stopping an engine and driving the vehicle by a motor generator at a light load, that is, the state where the amount of depression of an accelerator (hereinafter called an "accelerator opening") assumes a small value. In the above-mentioned method, the engine has to be automatically restarted when the accelerator opening is set to be more than the small value. At this time, while a portion of the driving force of the motor generator is used for running the vehicle, another portion is used to start the engine. Therefore, there is a decrease in the driving force due to the cranking load for the engine which gives the driver of the vehicle a feeling of excessive deceleration. Thus, a method is required to overcome the aforementioned problem by reducing the shock resulting from deceleration at the time of restarting the engine.
As a measure for preventing shock owing to deceleration, a technology is known to sweep up the engagement pressure of the clutch between the motor generator and the engine and to recognize a changing rate in the slight revolution of the motor generator caused by the increase in transmission force of the torque in the clutch engagement. Then, the output torque of the motor generator rises.
The cranking load generated when restarting the engine becomes a synthetic torque combining the torque of resistance generated by intake, compression and exhaust strokes in each cylinder, the torque corresponding to mechanical dragging resistance, and the torque for operating auxiliary units, such as an air conditioner, an alternator, a water pump and an oil pump with the inertia torque required to accelerate the stopped engine. Above all, the load generated by the intake/exhaust operations becomes a periodically oscillating torque as indicated by lines having different symbols respectively corresponding to the cylinders shown in FIG. 12. The total value of the aforementioned torque has a characteristic indicated by a solid line.
The actual cranking torque is characterized in that it is sharply increased to assume an excessive high torque value only at the start of revolution and then it assumes a substantially constant value as shown in FIG. 13. This is because inertia torque as a resistance against the revolutions, in turn, is caused to restrain the oscillation of the torque by flywheel inertia after the engine has been started. Therefore, the cranking torque which is needed in order to maintain revolutions at a predetermined speed may assume an average value.
Accordingly, a technology has been suggested to prevent generation of any load during intake/exhaust operations until the engine has been restarted to reach a certain number of revolutions in accordance with the characteristic of the cranking torque. Thus, the peak value of the torque load is reduced so as to decrease the cranking torque applied to the motor generator.
However, the latter technology requires reorganization of the engine as well as complicated control. Therefore, many problems have to be solved for a practical application. Meanwhile the former technology has a problem caused by the characteristic of the cranking torque. That is, the start-up characteristic of the cranking torque having the aforementioned periodically oscillating torque component is changed because the position of the peak is shifted from the crank shaft position at a stop state of the engine, as shown by a chain line, to that shown by the solid line of FIG. 14. As the timing of the generation of the peak torque is shifted as described above, the hydraulic pressure for engaging the clutch has to be changed to correspond to the foregoing capacity. Therefore, very precise control must be performed such that the amount of the increase in output torque from the motor generator is changed to correspond to the foregoing hydraulic pressure. This precise control cannot be performed by a simple control, such as a control using a map. Because the control cannot accurately estimate the oscillating torque component generated by the compression and expansion strokes in the cylinder, a shock is easily caused in an initial stage of starting the engine. What is worse, a satisfactorily high control speed cannot be realized.